Conventional steam crackers are an effective tool for cracking high-quality feedstocks that contain a large fraction of volatile hydrocarbons, such as ethane, propane, naphtha, and gas oil. Similarly, regenerative pyrolysis reactors, including reverse flow pyrolysis reactors (“RFR”), are also used for converting or cracking hydrocarbons and to execute cyclic, high temperature chemistry such as those reactions that may be performed at temperatures higher than can suitably be performed in conventional steam crackers.
Acetylene (or ethyne, HC≡CH) has long been recognized as one of the few compounds that can be made directly at high selectivity from methane pyrolysis but the conditions of that manufacture have placed it beyond commercial practicality for other than high cost, specialty production. Acetylene can be converted to a number of other desirable hydrocarbon products, such as olefins and vinyls. One of the biggest impediments to producing acetylene from methane or other hydrocarbon feeds has been the very high temperatures required to produce high-yield conversion of methane to acetylene. Many of the desired products that could be manufactured from the produced acetylene are today instead being produced via more economical processes, such as thermal cracking of higher molecular weight hydrocarbon feeds such as ethane, propane, naphthas, and gas-oil, in steam crackers. The higher molecular weight feeds generally crack at lower temperatures than methane. Equipment, materials, and processes were not previously identified that could continuously withstand the high (>1500° C.) temperatures required for methane pyrolysis. Pyrolyzing large quantities of methane into acetylene for conversion to olefins had been considered too costly and impractical due to the special types and costs of equipment that would be required. The developed commercial processes for producing acetylene have all operated at lower temperatures (e.g., <1500° C.) and are generally related to steam cracking of higher weight hydrocarbon feeds.
It is known that acetylene may be manufactured from methane in relatively small amounts or batches, using high temperature and short contact time in cyclical processes, yielding a mixture of acetylene, CO, and H2. Comprehensive discussions are provided in the Stanford Research Institute report entitled “Acetylene,” a Process Economics Program, Report No. 16, September 1966, and in the Fuel Processing Technology publication (42), entitled “Pyrolysis of Natural Gas: Chemistry and Process Concepts,” by Holmen, et. al., 1995, pgs 249-267. However the known processes are inefficient compared to commercial steam cracking, do not scale well, and are generally only useful for specialty applications.
Regenerative reactors, including reactors such as disclosed by Wulff (discussed below), are typically used to execute cyclic, batch-generation, high temperature chemistry. Regenerative reactor cycles typically are either symmetric (same chemistry or reaction in both directions) or asymmetric (chemistry or reaction changes with step in cycle). Symmetric cycles are typically used for relatively mild exothermic chemistry, examples being regenerative thermal oxidation (“RTO”) and autothermal reforming (“ATR”). Asymmetric cycles are typically used to execute endothermic chemistry, and the desired endothermic chemistry is paired with a different chemistry that is exothermic (typically combustion) to provide heat of reaction for the endothermic reaction. Examples of asymmetric cycles are Wulff cracking, pressure swing reforming (“PSR”), and other regenerative pyrolysis reactor processes.
The known art discloses that to efficiently obtain relatively high yields of acetylene from methane feed, such as in excess of 50 wt. %, preferably in excess of 75 wt. % acetylene from the methane feed, temperatures are required to be in excess of 1500° C. and preferably in excess of 1600° C., with short contact times (generally <0.1 seconds, with rapid quenching) to prevent decomposing the acetylene into coke and hydrogen. Such temperature and processes have largely been unattractive due to thermal, chemical, and mechanical degradation of the equipment utilized. Virtually any metal components directly exposed to such temperatures and stress due to cyclical temperature fluctuations, will be costly and unacceptably degrade over time. Although regenerative pyrolysis reactors are generally known in the art as capable of converting or cracking hydrocarbons, they have not achieved commercial or widespread use, due at least in part to the fact that they have not been successfully scaled well to a commercially economical size or commercially useful life span. These drawbacks to wide scale commercialization are due in large part to the inability of the equipment to adequately control and contend with the very high temperatures.
A related challenge has been in controlling how fuel and oxidant are combined during the regeneration or heating stage of the process. Inability to effectively control this issue contributes to the degradation of components and inefficiency at commercial scale. Due to uncontrolled exothermic reaction, the created high temperatures are difficult to position and contain for extended periods of time and lead to premature equipment failure.
Another challenge relates to materials stability at high temperature. Many prior art reactor materials undergo or become susceptible to chemistry alterations at the high process temperatures. The alterations lead to premature equipment degradation and potentially even interference with the process chemistry.
Complicating the issue stilt further for regenerative or cyclic reverse flow reactors has been the detrimental effects upon materials that are exposed to the large, cyclic temperature swings encountered during the process. Such effects become even more pronounced for high severity pyrolysis processes. A solution was proposed in a U.S. patent application filed Dec. 21, 2006, Ser. No. 11/643,541, entitled “Methane Conversion to Higher Hydrocarbons,” related primarily to methane feedstocks for pyrolysis systems, utilizing an inventive deferred combustion process. Although the disclosed process effectively controls the location of combustion, the internal reactor components must still contend with the severely high temperatures incurred in methane or other hydrocarbon pyrolysis for a commercially acceptable duration.
Typically, regenerative reactors include a reactor bed or zone comprising some type of refractory material in the reactive regions of the reactor, including the regions where the pyrolysis conversion or cracking reaction takes place and possibly also in those portions of the reactor that convey the reactants into and/or from the region where the reaction occurs. The refractory material comprising such reactive region may be a ceramic or other refractory material. Conventional regenerative reactors commonly deliver a stream of fuel, oxidant, or a supplemental amount of one of the reactants, directly to a location somewhere within the flow path of the reactor bed. In a deferred combustion regenerative reactor, the reactants are delivered separately into the region where they can mix and react at that location. The delivered reactants then are caused or permitted to exothermically react therein and heat the reactor media or bed in the reactive region of the reactor. Thereafter, direction of flow may be reversed or continue in the same direction, and the reacted components are exhausted and a pyrolysis feedstock, such as a hydrocarbon feed stream, preferably a vaporized feed, is introduced into the heated region of the media or bed. The pyrolysis feedstock is thus exposed to the heated media to cause heating and pyrolysis conversion of the reactor feedstock into a pyrolyzed reactor feed or product. The pyrolyzed reactor feed or product is then conveyed or otherwise removed from the reactive region of the reactor and quenched or cooled, such as in a quench region of the reactor system, to halt the pyrolysis reaction and yield a desired pyrolysis product, such as acetylene or ethylene.
Economics or feed availability may favor using lower cost feedstocks as feedstocks for regenerative pyrolysis reactors. As with steam crackers, regenerative pyrolysis reactors also are well suited for volatized or volatizable hydrocarbon feedstocks such as, by way of non-limiting examples, crude oil, heavy distillate cuts, contaminated naphthas and condensates, and atmospheric resids that are substantially free of nonvolatile components. Nonvolatile components, such as metals and other residual or substantially nonvolatizable components tend to “lay down” and build up in the reactor as ash, metals, and coke. Unfortunately, these economically favored feedstocks typically contain undesirable amounts of nonvolatile components and have heretofore been unacceptable as regenerative reactor feedstocks. Regenerative pyrolysis reactors do not have the flexibility to process such otherwise economically crack favorable feedstocks because, although coke can typically be burned off, deposits or buildup of ash and metals within the reactor cannot easily be burned or removed. A solution was proposed in a U.S. patent application filed Jun. 4, 2007, entitled “Pyrolysis Reactor Conversion of Hydrocarbon Feedstocks into Higher Value Hydrocarbons,” disclosing an inventive reverse flow regenerative reactor system suitable for pyrolyzing feedstocks heavier than methane, particularly those feedstocks that may contain or comprise non-volatizable components.
Chemical Economy and Engineering Review, July/August 1985, Vol. 17, No. 7.8 (No. 190), pp. 47-48, discloses that furnaces have been developed commercially for steam cracking a wide range of liquid hydrocarbon feedstocks using process reaction times in the range of 0.05 to 0.1 second. This publication indicates that the use of these furnaces permits substantial increases in the yield of olefins (i.e., ethylene, propylene, butadiene) while decreasing production of less-desirable co-products. GB 1064447 describes a process for production of acetylene from pyrolysis of methane and hydrogen (e.g., 1:1 to 39:1 H2:CH4) in an electrically heated reactor and quenching with a dry, oxygen-free gas stream. The maximum temperature is 1450° C. to 2000° C. (preferably 1450° C. to 1750° C.).
The “Wulff” process represents one of the more preferred commercial processes for generation of acetylene. The Wulff process includes a reverse-flow thermal pyrolysis process and began development in the 1920's. Various related processes operated commercially up to about the 1960s. These processes typically used feeds heavier than methane and thereby operated at temperatures of less than 1500° C. The most complete description of the Wulff process is provided in the Stanford Research Institute's “Acetylene”, Process Economics Program Report Number 16 (1966). Among the relevant patents listed in this report are U.S. Pat. Nos. 2,319,679; 2,678,339; 2,692,819; and 3,093,697, discussed above. It is believed that all commercial acetylene plants operated on feeds of ethane, naphtha, and/or butane, but that none have successfully operated on methane feeds. Wulff discloses a cyclic, regenerative furnace, preferably including stacks of Hasche tiles (see U.S. Pat. No. 2,319,679) as the heat exchange medium. However, to contain the location of the reaction heat generated by the exothermic combustion process, one of either the fuel or oxygen is introduced laterally or separately into the central core of the reactor where it mixes with the other reaction component therein. The other reaction component is preferably introduced through the reactor tiles to cool the reactor quench section. Thereby, combustion can occur in a targeted location within the reactor. However, the lateral introduction also exposes the lateral injection nozzles or ports to the combustion product, including the extremely high temperature needed to crack methane feeds. Degradation in nozzle performance, shape, and/or size consequently made it extremely difficult to control flame shape, temperature, and efficiency. Although some of the Wulff art disclose use of various refractory materials, a commercially useful process for methane cracking was not achieved utilizing such materials. Also, a further and more significant drawback of the Wulff process is that the laterally or separately introduced portion of exothermic reactant is not utilized for quenching the recuperation reactor bed. This imbalance in heat created and heat removed typically results in an expanding heat gradient through the reactor bed and corresponding changes in reaction timing, duration, and control. This situation creates significant difficulty with quenching the reaction at precisely the right time to produce the desired acetylene or other reaction product.
As discussed above, the technology of high-severity pyrolysis can result in high selectivity to acetylene that enables many dimensions of chemicals growth from natural gas and other hydrocarbon feeds. Analysis of the capabilities of reverse-flow reactors (RFR's) suggests that these reactors may achieve the desired reaction conditions only at extreme temperatures (≧1500° C. and in some cases even >1700° C.) in a cost effective manner. The aforementioned practical obstacles have impeded large scale implementation of the technologies. Materials availability for such high temperature is one of the most critical issues in design and operation of large-scale, commercial, high-productivity, RFR's.
Due to ultrahigh temperatures involved in RFR's, only ceramic components have the potential to meet the materials characteristics needed in such an aggressive application. The American Society for Testing and Materials (ASTM) defines a ceramic article as “an article having a glazed or unglazed body of crystalline or partly crystalline structure, or of glass, which body is produced from essentially inorganic, non-metallic substances and either is formed from a molten mass which solidifies on cooling, or is formed and simultaneously or subsequently matured by the action of the heat.” Ceramics components generally can be categorized in three material categories: engineering grade, insulation grade, and refractory grade.
The term “engineering grade” has been applied to ceramic materials which typically have very low porosity, high density, relatively high thermal conductivity, and comprise a complete component or a lining. Examples include dense forms of aluminum oxide (Al2O3), silicon nitride (Si3N4), silicon carbide (SiC), silicon aluminum oxynitride (SIALON), zirconium oxide (ZrO2), transformation-toughened zirconia (TTZ), transformation-toughened alumina (TTA), and aluminum nitride (AlN). These materials usually possess high strength and toughness, which have been dramatically improved to the degree that ceramics are now available that can compete with metals in applications previously thought impossible for ceramics. Strength is a measurement of the resistance to formation of a crack or structural damage in the material when a load is applied. Toughness is a measurement of the resistance of the material to propagation of a crack or extension of damage to the point of failure. For instance, engineering grade Al2O3 and SiC are commercially available with a strength of over 345 MPa, and Si3N4 and TTZ are available with strengths above 690 MPa (100 kpsi). Some TTZ materials have toughness around 15 MPa·m1/2, which is an order of magnitude higher than that of conventional ceramics. Even though engineering grade ceramics have superior strength and toughness at relatively low temperatures, they are relatively poor in thermal shock resistance and many grades, such as but not limited to borides, carbides, and nitrides are not chemically stable at high temperature. Many are also not suitable for use at the high temperatures encountered with some pyrolysis reactions.
The second category of ceramic materials is insulation grade ceramics, which are typified by relatively high porosity. Many may have fibrous crystalline grain structures and are more porous than engineering grade ceramics, have lower density, and have lower thermal conductivity than engineering grade ceramics. Insulating monolithic ceramics and composite ceramics are often fabricated into various forms such as rigid boards, cylinders, papers, felts, textiles, blankets, and moldables. Many are primarily used for thermal insulation at elevated temperatures, such as up to 1700° C. A broad range of porosities and pore sizes can be produced, depending on the intended application, but in general, insulation grade ceramics tend to be relatively porous as compared to engineering grade ceramics. Porous ceramics have many open or closed internal pores that provide the thermal barrier properties. Often, porous ceramics, such as having porosity of for example, greater than 50 vol. % and commonly even in excess of 90 vol. %, are used for thermal insulation where extremely low thermal conductivity (<0.08 W/m·K) is required. However, insulation grade ceramics usually lack the structural strength and functional toughness needed for the internal components of RFR's that are directly exposed to the pyrolysis reactions and flowing, hot reactants or products. Insulation grade ceramics typically are recognized as having a flexural strength or toughness of less than about 4 Kpsi (27.6 MPa) and often of less than even 1 Kpsi (6.9 MPa). Also, the insulation properties of porous ceramics may also tend to degrade over time as the relatively large pores may tend to fill with coke accumulation.
The third general category of ceramic materials is refractory grade ceramics. Many refractory grade ceramics typically have porosity, strength, and toughness properties that tend to be intermediate to these properties in engineering grade and insulation grade. Refractory grade ceramics typically have higher thermal shock resistance properties and maximum use temperatures than most insulation grade and engineering grade ceramics. However, refractory grade ceramics have other properties that may vary as compared to engineering and insulation grade ceramics that distinguish each of the various refractory grade ceramics from the engineering and insulation grade ceramics, and from each other. Each of these other properties must also be considered when selecting a refractory grade ceramic for a particular application. Some of the other relevant properties or characteristics include but are not limited to maximum use temperature, thermal conductivity, modulus of rupture, modulus of elasticity, electrical resistance, average grit size, density, porosity, and purity. The maximum use temperature is the highest temperature to which refractory ceramics can be exposed without degradation. Thermal conductivity is the linear heat transfer per unit area for a given applied temperature gradient. The modulus of rupture (MOR) or cross-break strength is the maximum flexural strength that refractory ceramics can withstand before failure or fracture occurs. Young's modulus or the modulus of elasticity is a material constant that indicates the variation of strain produced under an applied tensile load. Average grit size measures the size of individual grains or crystals within the microstructure of a polycrystalline ceramic material. Density is the mass per unit of bulk volume. Purity is the percentage, by weight, of major constituents.
As compared to insulation grade ceramics, refractory grade ceramics tend to be stronger across broader temperature ranges. Refractory grade ceramics also generally tend to be more resistant to thermal shock than insulation or engineering grade ceramics. However, while some refractory grade ceramics tend to be somewhat inert or chemically stable at elevated temperatures, some refractory grade ceramics become chemically and/or structurally unstable at elevated temperatures, rendering them unsuitable for applications exposed to chemical reactions. Exemplary chemically and/or thermally unstable ceramics include certain silicas, borides, carbides, and nitrides. Also, some refractory grade ceramics are also known to possess lower thermal conductivities and coefficients of expansion than certain other refractory or engineering grade ceramics. Others have variations of these properties. Refractory grade ceramics are also known to undergo alterations in crystalline structure at elevated temperatures. Such alterations can result in changes in bulk volume which can result in production of stress fractures and/or cleavage planes which can reduce the material's strength. Some exemplary, common high temperature refractory grade materials include but are not limited to magnesia (MgO), lime (CaO), and zirconia (ZrO2). However, the studied art does not teach preferred crystalline structure or composition for particular reactor furnace uses.
The reviewed art is also void of teaching how to prepare or select a material having a range of properties that are suitable for use in constructing a furnace for performing substantially continuous, cyclical, high temperature pyrolysis chemistry. Also, materials testing methods commonly applied to metals and polymers are frequently less useful for testing ceramics. The available tests provide only a limited picture of the total performance limits of any particular ceramic. Further complicating the ceramic material selection process is the complexing fact that, like metals and polymers, the performance of a ceramic is also a function of temperature, with dependent changes in properties such as brittleness, elastic, plastic and viscoplastic deformation, hardness, fatigue, corrosion resistance, and creep resistance. Other important performance factors include but are not limited to thermal shock resistance, thermal expansion, elastic modulus, thermal conductivity, strength, and fracture toughness.
The identified prior art pertaining to refractory materials for high-severity hydrocarbon pyrolysis dates primarily to the 1960's and earlier. However, that art merely occasionally provides generalized lists of some exemplary materials such as ceramics, alumina, silicon carbide, and zircon as reactor materials. These sparse, non-specific disclosures left the art largely incapable of providing a large-scale, commercially useful reactor or reactor process. The teachings of the art was only effective for enabling relatively small scale specialty applications that see vastly inferior use as compared to large scale processes such as hydrocarbon steam cracking. The identified art is void of teaching or providing a refractory ceramic material that has the complex set of properties that are required for extended use in the reactive or other most-demanding regions of a high-severity (≧1500° C.) pyrolysis reactor for the commercial production of acetylene and/or olefins. The art needs a refractory material that can endure prolonged exposure to high severity temperatures, substantial temperature swing cycles, cyclic flows of combustion and reaction materials, and concurrently provide the needed structural integrity, crystalline stability, relatively high heat transfer capability, and chemical inertness in the presence of high temperature chemical reactions that is required for large scale, high productivity applications. Lack of materials availability and selection criteria for identifying the materials for use in the reactive and most severe temperature regions of a reactor system is one of the most critical remaining issues in design and large-scale operation of such reactors.